To improve the methods for radioiodine testing in thyroid cancer, the effect of time after stopping thyroid hormone replacement was evaluated. An interval shorter than heretofore employed may be adequate. In 50 consecutive thyroid nodules, fine-needle aspiration cytology was used to diagnose malignant lesions. A diagnosis of malignant by cytology was correct in 78% and a negative diagnosis was correct in 94%. In patients with suspicious cytology, 40% had cancer. Lithium was shown in one patient to decrease the rate of release of 131I from thyroid cancer metastases, and thus may provide a means to increase the therapeutic/toxic ratio in 131I therapy for this disease. Additional cases of hyperthyroidism due to inappropriate TSH secretion were studied. Patients with pituitary tumors had elevated alpha-subunit levels and autonomous TSH secretion, whereas patients with "resistance" syndromes had appropriate alpha secretion and responsiveness to TRH and thyroid hormones. BIBLIOGRAPHIC REFERENCES: Gershenogorn, M.C., Larsen, P. R. and Robbins, J.: Radioimmunoassay for serum thyroxine-binding globulin: Results in normal subjects and in patients with hepatocellular carcinoma. J. Clin. Endocrinol. Metab. 42:907-911, 1976. Gershengorn, M. C., Izumi, M. and Robbins, J.: Use of lithium as an adjunct to radioiodine therapy of thyroid carcinoma. J. Clin. Endocrinol. Metab. 42: 105-111, 1976.